Transmitters which can be attached to or worn by a person so the person may be detected within a monitored area are well known. Such transmitters are usually encased within a housing having a strap attached thereto by which the transmitter is secured about a limb or other body part of the person. A monitoring unit is provided within the monitored area to receive a transmitted signal or message from the transmitter and the monitoring unit generates an alarm when the signal from the transmitter is no longer received. The transmitter usually includes an identification code in the transmitted message to make it more difficult for the confinee to use a second transmitter to provide the signal to the monitoring unit when the confinee leaves with the transmitter strapped to his limb.
When the monitored area is a residence which is not typically secured by guards or limited access devices, a device which detects and indicates removal of, or tampering with, the transmitter is needed. Otherwise, the confined person could remove the transmitter, leave it within the monitored area and escape from the area without his absence being detected. A transmitter which provides a removal or tamper-indicating signal to the monitoring unit is shown in U.S. patent application No. 07/343,814 entitled "Remote Confinement System With Timed Tamper Detection Reset," filed on Apr. 26, 1989 and is assigned to the assignee of the present application. The entire disclosure of the referenced application is herein expressly incorporated by reference.
The transmitter of the referenced application has a circuit which monitors the current through a conductive strap used to secure the transmitter to the confinee to detect tampering with the strap. When the current is no longer sensed due to tampering, such as cutting the strap or disconnecting the housing from the strap, a tamper-indicating bit is set and transmitted in a message to the monitoring unit. The transmitted message continues to include the set tamper-indicating bit until a predetermined time period has elapsed following detection of the tamper event. At the expiration of the predetermined time period, the tamper-indicating bit is reset and transmitted in subsequent messages to the monitoring unit to thereby indicate a "no tamper" condition. When the monitoring unit receives a message containing a set tamper-indicating bit, it determines whether an alarm should be sent to a central monitoring station manned by supervisory personnel. If an alarm is sent to the central monitoring station, the supervisory personnel report the alarm to the officer responsible for the confinee, such as a parole officer or the like.
While verifiable confinement of an individual within his home reduces government expenses since housing and guards are not required, it is preferable that the home arrest system also permit the confinee to leave the monitored site to go to his job. Such a system has the social advantage of allowing the confinee to provide for himself and his family.
At work, while out of range of the monitoring unit, the confinee may cause a tamper event by removing the transmitter from his limb since the strap must be opened to effect removal. Such a tamper event occurring at the workplace goes undetected by the monitoring unit at the monitored site. Upon return to the monitored site, the confinee could leave the detached transmitter with the strap closed in the monitored area so the monitoring unit begins to receive transmissions from the detached transmitter (with closed strap). The confinee may now leave the monitored area without detection. The transmitter of the above referenced patent application prevents this from occurring by latching the tamper indicating bit in the set state for a predetermined time which is long enough to provide a tamper-indicating message to the monitoring unit upon the return of the individual to the monitored site following an excused absence, that is, for a period of time which exceeds the permissible length of the excused absence interval. In a preferred version of the referenced transmitter, this time period, i.e., herein termed the "tamper alert period," is approximately 18 hours.
Unfortunately, such a lengthy tamper alert period presents a problem when the transmitter is initially secured to the individual. At the time of initial installment of the transmitter on the confinee at the monitored site, the strap is opened and then closed about the individual's limb resulting in the periodic transmission of messages containing a set tamper-indicating bit to the monitoring unit for the entire tamper alert period. In the home arrest system of the referenced transmitter, the monitoring unit ignores the receipt of the set tamper-indicating bit in the transmitted messages and does not relay tamper messages to a central monitoring station for a second time period. The monitoring unit initiates this second time period which is longer than the tamper alert period following the initial powering of the monitoring unit which is usually close in time to the powering of the transmitter. If the strap is closed at the expiration of the tamper alert period, the transmitter transmits messages containing a reset tamper-indicating bit and the monitoring unit terminates the timing of the second time period. If the monitoring unit receives a transmitted message thereafter containing a set tamper-indicating bit, it sends an alarm to the central monitoring station. If the confinee cuts the conductive strap and leaves it in the monitored area while the first and second time periods are being timed, the transmitted tamper-indicating messages are ignored by the monitoring unit until the expiration of the second time period. Upon receipt of the next tamper-indicating message, the monitoring unit generates an alarm in response to the tamper-indicating messages. This gives the confinee an opportunity to leave the monitored area without detection during the second time period.
If the tamper alert period and second time period are shortened so both time periods expire before the installing officer leaves within a relatively short time, then a tamper during an excused absence, such as a work period, would go undetected because the transmission of messages containing the set tamper-indicating bit caused by the tamper at the work place would terminate prior to the return of the confinee to the monitored site. A transmitter that can be worn by a confinee is needed which indicates an unauthorized tamper just following securement of the transmitter to the confinee, yet provides a tamper indication for a period of time sufficient to provide detection of tampering during excused absences from the monitoring site. Such a transmitter would eliminate the need for the second period in the monitoring unit which prevents transmission of tamper messages to the central monitoring station following installation.
As disclosed in the incorporated patent application, another solution to this limitation has been the use of external reset devices which reset the tamper-indicating circuit when brought in proximity to the transmitter. Access to such manual reset devices must be closely monitored to prevent the device or a duplicate from being obtained by someone who would use it to prevent the monitoring unit from detecting a tamper with the transmitter.
Another limitation of home arrest systems arises from the varying sizes of the houses in which the confinees live. Affluent confinees, such as pre-trial detainees who are major drug dealers, may have homes that cover several thousand square feet of area. Other confinees who are poorer may live in houses or apartments having substantially less than one thousand square feet. If a single range transmitter having a range sufficient to only cover the smaller house is used in a large house, certain areas of the larger home are beyond the monitored area. As a consequence, in a large home the monitoring unit generates an alarm when the confinee leaves the "effective" monitored area and ventures into an unmonitored area of the home. For example, if the den is in the monitored area of a large home and the bedroom is not, then leaving the den and retiring to the bedroom would generate an alarm. Conversely, should the transmission range of the transmitter be sufficient to cover a large house, a confinee within a much smaller home would be able to go to a nearby street or residence, which usually violates the terms of the home arrest, without the monitoring unit generating an alarm. What is needed is a transmitter having a transmission range that can be adjusted to the size of the monitored area at the time it is secured to the confinee's limb and can indicate attempts by the confinee to alter the transmission range thereafter.